1. Technical Field
The present invention relates generally to controlled access and more particularly to a keyless entry system which is especially adaptable for application in vehicles.
2. Background Art
A keyless entry system wherein a card type portable transmitter (substantially the same size as a credit card, with a thickness of about 3 mm) is carried by the user is well known in the art. The user pushes a request switch button installed on a driver's door to communicate between the portable transmitter and a door lock system to lock or release a door lock without a mechanical key. This keyless entry system can save a user taking out a mechanical key to lock or unlock a door at every access, to an automotive vehicle for example.
A U.S. Pat. No. 4,794,268, entitled "AUTOMOTIVE KEYLESS ENTRY SYSTEM INCORPORATING PORTABLE RADIO SELF-IDENTIFYING CODE SIGNAL TRANSMITTER" by Kinichiro Nakano et. al., assigned to NISSAN MOTOR CO., LTD, discloses a keyless entry system using a card type transmitter. Such a keyless entry system, as applied to a vehicle, includes a door lock control system disposed within the vehicle. Depression of a request switch button disposed on a door handle for example causes a request code signal provided in the door lock control system to be transmitted from an antenna as a starting signal to a portable transmitter. In the portable transmitter, the received request code signal is collated with an identifying code signal stored in a memory of the transmitter. If coincidence between the code signals is indicated, the portable transmitter outputs an owner permanent code signal to the vehicle. The door lock control system receives the owner permanent code signal to compare it with the vehicle permanent code signal, operating an actuator for locking or unlocking the driver's door when both permanent code signals are coincident with each other.
However, the conventional keyless entry system uses a fixed code signal to communicate between vehicle and transmitter. Thus, if communication signals were received by an unauthorized person using a high sensitivity receiver for example and a transmitter capable of transmitting the same signal is used, the door lock may be easily released by an unauthorized person without need of the regular portable transmitter.